The invention concerns a method for producing containers filled with a liquid filling material from thermoplastic preforms, and a device for producing the containers filled with the liquid.
The production of containers by blow molding from preforms made of a thermoplastic material is well-known, for instance from preforms made of PET (polyethylene terephthalate), whereas the preforms are conveyed to various machining stations inside a blow molding machine (DE-OS 43 40 291). Typically, a blow molding machine includes a heating device for tempering or preheating (thermal conditioning) of the preforms as well as a blow molding device with at least a blow molding station, in which area the preform previously tempered is expanded biaxially or multiaxially to become a container. The expansion takes place with the aid of a compressed gas (compressed air) as a pressure medium, which is introduced into the preform to be expanded with a forming pressure. The process-related sequence of such an expansion of the preform is explained in document DE-OS 43 40 291.
The basic construction of a blow molding station is described in DE-OS 42 12 583. The possible tempering methods of the preforms are explained in DE-OS 23 52 926.
According to a typical further processing treatment, the containers produced by blow molding are conveyed to a subsequent filling device and then filled with the product or filling material provided. A separate blow molding machine and a separate filling machine are thus used. It is also known to regroup the separate blow molding machine and the separate filling machine to obtain a machine block, i.e. to obtain a blocked blow molding and filling device, whereas the blow molding and the filling continue to be performed at distinct machine components and chronologically one after the other.
It has been moreover already suggested, to produce containers, in particular in the form of bottles, from thermally conditioned or preheated preforms and thereby to fill them simultaneously with a liquid filling material which is conveyed as a hydraulic pressure medium for expanding the preform or for shaping the container with a forming and filling pressure so that the respective preform is transformed into the container at the same time as the filling process. Such methods, at which simultaneous forming and filling of the respective container takes place, can be designated as a hydraulic transformation method or hydraulic formation of containers.
During the formation of the containers from the preforms by the filling material, i.e. by using the filling material as a hydraulic pressure medium, only one machine is necessary to form and fill the containers, which in turn exhibits an increased complexity. The first experimental results with such devices indeed show that the quality of the produced containers is still significantly lower than the quality of conventionally produced blow formed containers. The cause is among other things that a multiplicity of process parameters which are available when performing the usual blow molding process, are either absent during the hydraulic formation of containers or cannot be exploited as yet.
There is a particular problematic with the hydraulic formation of containers inasmuch as any contamination should be avoided for the respective forming and filling station or of the mould constituting said station, which mould is similar to a blow mould of a blow molding machine for the production of containers from thermally conditioned preforms by blowing with a compressed gas. Especially in the case of a full or partial carbonation of the filling material, there is in a particular extent the danger of a contamination of the respective forming and filling station through losses of filling material, in particular when lowering the inside pressure of the container, i.e. when relieving the container from the quite high forming and filling pressure down to the ambient pressure. Such losses of filling material are conditioned in particular by a massive foaming during the relieving process so that the simultaneous forming and filling of containers by using preforms and by using the filling material as a pressure medium (hydraulic transformation technique), in particular for CO2 containing products could not be contemplated so far.